It is a simple concept: the forage (grass, forbs [wildflowers], shrubs)
consumed by domestic livestock is not available as food and cover for native
wildlife--species that are important to healthy ecosystems, admired by wildlife
enthusiasts, and prized by hunters. Range managers use the rather imprecise
animal unit month or AUM to measure and allocate forage. An AUM is the amount
of forage necessary to sustain a cow and calf for one month (approximately
650 pounds, although some estimates are more, between 800-1000 pounds 2).
Below are generally accepted AUM equivalents.

Native Wildlife

Animals Per AUM 3

Domestic Livestock

Animals Per AUM 4

Bighorn Sheep

6.9

Cow

1

Pronghorn

10.8

Bull

1.25

Mule Deer

7.8

Horse

1.25

Elk

2.1

Goat

5

Bison

0.8

Sheep

5

Moose

1.2

The threatened desert tortoise eats less vegetation in one year than a cow
eats in one day. 5